The invention relates to an inductively operating absolute length and angle measuring system, in which a coil structure and the corresponding evaluation electronics move in a scanning head along the measuring path relative to the absolutely encoded scale and measures the position, in accordance with DE 19803249 A1 discussed below.
Generally, two types of such devices for position measuring are known, namely the incremental measuring arrangement in which a periodic division is scanned and, by adding or subtracting measuring increments, the relative position scale/scanning unit is computed relative to a reference position, and the absolute measuring arrangement is obtained in the scale/scanning unit for any relative position for a combination of signals generated in the scanning unit which is unique for the entire measuring range.
It is apparent that an absolute measuring device, which when an axis of movement of a plant is switched on, determines the position directly without having to carry out a reference trip to a known position, and additionally, advantageously facilitates in the case of an interruption, a further secure operation of the plant.
Generally, because of the simpler technical conversion, the incremental measuring systems are more widely used and offer higher resolutions and accuracies. For both measuring methods, measuring devices are known which operate on optical, magnetic, capacitive or inductive physical principles.
The invention relates to the inductively operating measuring systems which, compared to optical systems, are significantly less sensitive to external ambient factors and, can achieve a higher accuracy as compared to the magnetic or capacitive systems. For this reason, only the following variation be considered with respect to the state of the art.
EP 1164358 B1 describes a highly accurate inductive incremental measuring system in which a periodic division is scanned be a compensated coil structure and resolutions in the range of <1 μm can be achieved. The measuring system operates on an incremental basis and, therefore, for initializing a drive system, a reference trip is required and, in case of an interruption of the operation, cannot recognize its position assumed last after the operation has started again, and for this reason, may lead to damage to machines and/or persons because of an undesired movement of the axis.
For the family of devices that the invention deals with, basically two methods are used as absolute position measurement. The so-called Nonius method is based on the determination of the phase difference between at least two periodic incremental divisions of different division periods extending along the measurement path. Under limiting conditions concerning resolution of the position measurement and maximum achievable measuring range, a specific phase difference occurs only once and, therefore, each phase difference can be assigned in the electronic evaluation circuit to a specific absolute position.
Additional incremental divisions of further different periods or periods designed to be longer, can facilitate compromises between higher resolution and greater absolute measuring range.
DE 69925353 T2 shows for an absolute measurement device according to the Nonius principle with three incremental divisions of different periods that the achievable ratio of division periods/measurement range in the embodiment paragraph [048] is approximately 2.5 mm/325 mm, and in the embodiment paragraph [0135] approximately 5 mm/2677 mm. Because of the long division period, this means an achievable resolution of 10 μm for a maximum measuring range of only 2,627 mm; accordingly, it is very limited.
The second known method for the absolute positioning determination is the so called “Quasi Random Code” division (FIG. 1), in which a division (T1) with alternating ranges of different lengths in measuring direction is constructed as a plurality of a division period “λ” in such a way that, by scanning “N” adjacent division periods a code word is created which has a length of “N”-Bit and which occurs once for the entire measuring range and can be assigned to a specific absolute position Xi by recoding in a “Look Up Table” (LUT).
FIG. 1 shows an example from the state of the art in which four sensor cells (photo elements) scan an encoded division of an LED in passing light and thereby create a four Bit word for the illustrated relative position, to which is assigned after recoding in the Look Up Table (LUT) a length position Xi. In this measuring arrangement the signals having amplitude “A” have an offset “O” and, for the logical signal formation the signals have to be evaluated at a level “P.”
Such devices operating on the optical principle are widely used and are illustrated, for example, by the company Heidenhain in brochure No. 571 470-14.-30-06-2007.
It is generally known that optoelectronic devices are susceptible to contamination, condensation, water, or other foreign bodies which could impair scanning the light ray.
DE 19803249 A1 describes an inductive measuring device with absolute position measuring which, in addition to variations operating according to the Nonius method, also shows absolutely encoded devices. In this case, as described in FIG. 21 in the fourth embodiment, an encoded division of individual bit-related receiver coils is scanned, which are excited by a single emitter coil which surrounds (452) all individual receivers for the generation of signals. In this sensor arrangement, the induction has within the surface of the emitter coil a high gradient and, as a result, the amplitude of the generated signals significantly varies from bit to bit in the individual bit-related receiver coils (457) in dependence on the distance to the emitter coils within the emitter surface.
These aspects are recognized the reference, however, the solution proposed above to increase the emitter coil surface, so that emitter coils are arranged further from the receiver coil, reduces minimally the lack of uniformity of the induction of the individual receiver coils, but decisively reduces the total exciting field strength in the emitter coil surface and, thus, the signal gain, so that the electrical evaluation of the signals becomes even more difficult, especially in the case of a change of the air gap between sensor and scale, which is practically unavoidable for an industrial application.